Well, lots of things could have caused it to crack. If you dont already know what did it, you probably never will. How bad is the crack? If it's not like a huge hole, but just a thin crack, you dont really need a new tank. You can go into ANY auto supply store and get Gas Tank repair gel/sealant. You apply it, wait ONLY 5 minutes, and it hardens right away. Its sandable AND paintable.
Otherwise, you can try Craigslist, E-bay, Motorcycle shop or a local used motorcycle parts place.
Good luck!